Prune juice is currently produced by one of two methods. The "diffusion" method involves leaching the soluble solids from dried prunes by successive 2-4 hour hot water extractions for a total time of about 12 hours. The combined extracts are then concentrated to 18.5% solids or greater to produce the finished juice. In the "disintegration" method, the prunes are disintegrated by 2-3 hours of cooking in boiling water. The juice, containing 18.5% soluble solids or greater, is then separated from the disintegrated pulp and the pits by pressing and/or filtration.
The diffusion process is time-consuming and gives relatively low yields of clear juice with a moderate viscosity. The disintegration process is much faster and gives higher yields of juice with a relatively high viscosity, but is much more energy intensive.
In the early 1970's a modification of the disintegration process focused on the use of pectinase enzymes was proposed. Pectinase treatment of the partially disintegrated prunes shortens the cooking time and results in energy savings. The enzyme treatment also improves filterability of the disintegrated prunes. However, pectinase treated juice has a very low viscosity. Because high viscosity is important to the organoleptic quality of prune juice, the modified process (which produces a low viscosity juice) has not met with great success in commerce.
Another modification to the current practice of the disintegration process that was proposed (along with the enzyme treatment step) is separation of the juice from the prune pulp residue by centrifugation rather than filtration. The centrifugal separation procedure is appealing since it does not require addition of a filter aid and the residue does not contain significant levels of the filter aid. Unfortunately, good yields cannot be obtained by centrifugation of prunes disintegrated in the normal fashion. Centrifugal separation is only feasible for recovering the low viscosity juice from prune mixtures that have been treated with pectinase, and, therefore, this too has not been adopted in commercial practice.